It is well known to use an OTDR to determine or monitor loss characteristics of an optical fiber path in an optical communications system. With increasing use of optical communications, and with extension of optical fiber communications to subscribers' premises, there is an increasing need for such monitoring in a convenient and effective manner. In particular, it is desirable to facilitate central monitoring of the characteristics of optical fibers leading to many subscribers' premises from a central location, without gaining access to the remote ends of the fibers, as is done with automatic loop testing on conventional copper subscriber lines. In So et al. U.S. Pat. No. 4,911,515, issued Mar. 27, 1990, entitled "Optical Fiber Communications System With Optical Fiber Monitoring", and assigned to Northern Telecom Limited, there is described an OTDR arrangement which facilitates such central monitoring.
Optical fibers which are currently used in optical communications systems are predominantly "single mode" fibers, in that one mode (LP.sub.01) is propagated over large distances with relatively little attenuation or loss, whereas other, higher order, modes are heavily attenuated over such distances, so that only the one mode is effectively propagated over a long length of an optical fiber communications path. At any discontinuity in an optical fiber path, such as occurs at an optical fiber splice or optical connector, a portion of the light travelling in the fiber core is lost, the majority of the lost light being transferred from the LP.sub.01 mode to the LP.sub.11 mode. As is known, the LP.sub.11 mode travels faster than the LP.sub.01 mode, but is relatively quickly attenuated.
A problem arises, however, if two discontinuities occur only a relatively short distance apart. In such a situation, at the second discontinuity the LP.sub.11 mode can have sufficient power that some of it can be converted back into the LP.sub.01 mode, this recoupled light then being propagated along the fiber and interfering with the desired LP.sub.01 mode signal, with which it is no longer synchronized due to the different velocities of the LP.sub.01 and LP.sub.11 modes between the two discontinuities. The net effect of this is modal interference, or modal noise, which appears as a wavelength dependent loss of the optical fiber path.
Such relatively closely spaced discontinuities may occur in a variety of situations, for example with repeated splicing of an optical fiber, with the use of optical fiber patch cords, or with the use of certain types of optical connectors which incorporate a short length of fiber to facilitate field assembly of the connectors. Generally, in any situation where there is less than a few meters of fiber between two successive discontinuities, there is a potential for modal interference as described above.
OTDRs conventionally used for optical communications systems have a resolution of, at best, about 0.1 m, and accordingly are unable to resolve between discontinuities closer apart than this, merely indicating the combined loss of the two discontinuities as though there is only a single discontinuity. Accordingly, conventional OTDRs fail to assist in determining the existence and location of such discontinuities. It should be appreciated that the combined loss of two closely spaced discontinuities may not be particularly great, but the modal interference may be sufficient to cause significant degradation of the optical communications path, leading to excessive transmission error rates. Furthermore, it should be appreciated that the wavelength dependent nature of the loss due to this modal interference may mean that the errors are intermittent or vary with time, due to small changes in the wavelength of the light being transmitted via the fiber.
An object of this invention, therefore, is to provide an improved OTDR which facilitates the detection of such closely spaced discontinuities.